French patent number 2 588 319 describes a method of accurately controlling the throughput of a metering pump by acting on three parameters of the electricity power supply to a reversible synchronous motor, namely: the winding to which the electricity is applied, thereby determining the direction in which the motor rotates; the time during which the electricity is supplied, thereby determining the length of the half-stroke of the pumping member; and the total time of a go-and-return cycle of said member, thereby determining the pump rate. In said method, a microprocessor is used which receives input data corresponding to the desired throughput expressed as a fraction of the maximum throughput of the pump or else expressed in the form of two items corresponding respectively to the desired stroke and to the desired rate, and it then selects appropriate motor control parameters on the basis of said received data with the selected values being taken from values contained in its memory, and it then controls the motor as a function of the parameters it has selected.
It has been observed that the method described requires the use of very high quality pumps, since the values stored in the memory of the microprocessor imply that there is very little dispersion in the operating parameters of different pumps, i.e. the pumps need to be manufactured with a great deal of care to ensure that desired precision is obtained to within better than 1%.
There also exist less sophisticated metering pumps in which throughput is controlled firstly by controlling the rate of the pump by means of electronic regulation, and secondly by controlling its stroke by acting mechanically on the components of its transmission by means of abutments for the displacement of the transmission components or by using servo motors to drive said components.
The present invention seeks to extend the advantages of throughput control that does not act on mechanical components by means of stroke-limiting members which are always difficult to adjust and liable to come out of adjustment, and which does not make use of servo motors, while nevertheless using means which are simplified in comparison to the means described in the above-mentioned French patent.